1. Field of the Invention
This invention relates to a method and apparatus for creating natural gas reservoirs within geopressured or hydropressured aquifers which contain appreciable quantities of natural gas and producing the natural gas from the aquifers and simultaneously producing hot water for extraction of thermal energy, if desired.
2. Description of the Prior Art
Hydropressured aquifers are porous, permeable water bearing formations in which the interstitial fluid pressure reflects the weight of the superincumbent water column, unconfined above, and open to the atmosphere. The depth-pressure gradient is mainly a function of the dissolved solids content of the formation water, and may range from about 0.3 to about 0.5 pound per square inch per foot of depth.
Geopressured aquifers are not open to the atmosphere, having been compartmentalized by faulting, and their fluid pressures reflect a part of, or all of the weight of the superincumbent rock deposits. The depth-pressure gradient is mainly a function of rate of leakage, or fluid escape, from the aquifer system, and may range from about 0.5 to about 1.0 pound per square inch per foot of depth.
Geopressured aquifers exist along the Gulf Coast of the United States and in many other places throughout the world where sedimentary deposits have been rapidly buried. Due to the high pressures found in geopressured aquifers, if a well is drilled into the aquifer, water will flow to the surface of the ground in artesian fashion.
Natural gas may be present in geopressured or hydropressured aquifers in any of these forms:
(1) Gas dissolved in the water,
(2) Free gas dispersed in water within the rock pores, and
(3) A free gas phase present within the rock pores and separate from the water.
The natural gas contained in aquifers is commonly 95-98% or more methane.
The conventional method of producing hydrocarbon fluids from oil and gas wells is designed to restrict the flow rate so as not to reduce drastically the pressure in the vicinity of the production well and draw water into the well. In order to do this, the well completion is in a zone above the oil-water or gas-water contact. Conventionally, gas wells cease production when water invades the area near the well bore and appreciable quantities of water are produced with the gas.
Publications which relates to the background of this invention and which are referred to herein are as follows.
1.--Reeves, "Italian Oil and Gas Resources," American Association of Petroleum Geologists Bull., v. 37, no. 4, Pp. 625-628, 1953
2.--Buckley, et al, "Distribution of Dissolved Hydrocarbons in Subsurface Waters," Pp. 850-882, "Habitat of Oil", L. C. Weeks Ed., American Association of Petroleum Geologists Special Publication, 1958
3.--Marsden and Kawai, "Suiyosei-Ten'nengasu, A Special Type of Japanese Natural Gas Deposit," American Association of Petroleum Geologists Bull., v. 49, no. 3, Pp. 286-295, 1965
4.--Hammerlindl, "Predicting Gas Reserves in Abnormally Pressured Reservoirs," SPE preprint 3479, 6 p., 4 figs: Society of Petroleum Engineers of AIME, Dallas, Tex., 1971
5.--Perry, "Statistical Study of Geopressured Reservoirs in Southwest Louisiana," SPE preprint 3888, 3 p., 4 tables, 6 figs: Society of Petroleum Engineers of AIME, Dallas, Tex., 1972
6.--Sultanov, et al, "Solubility of Methane in Water at High Temperatures and Pressures," Gazovaia promphlennost, v. 17, no. 5, Pp. 6-7, May 1972
7.--Jones, "Geothermal and Hydrocarbon Regimes, Northern Gulf of Mexico Basin," Pp. 15-89 Proceedings of the First Symposium on the Geopressured Geothermal Resources of the Gulf Basin, Austin, Tex.: The University of Texas at Austin, 1975
8.--Jones, "Natural Gas Resources of the Geopressured Zones in the Northern Gulf of Mexico Basin," Pp. 17-33, Natural Gas from Unconventional Geologic Sources, Board on Mineral Resources, Commission on Natural Resources, National Academy of Sciences, Washington, D.C., 1976
9.--Isokrari, "Natural Gas Production from Geothermal Geopressured Aquifers," SPE preprint 6073, 9 p., 6 tables, 18 figs: Society of Petroleum Engineers of AIME, Dallas, Tex., 1976
10.--Randolph, "Natural Gas from Geopressured Aquifers," SPE preprint 6826, 8 p., 1 table, 8 figs: Society of Petroleum Engineers of AIME, Dallas, Tex., 1977
Commercial development of natural gas dissolved in saline formation waters began in the Tokyo Bay region of Japan in 1931, from wells ranging up to 400 m (1312 ft) deep, and is now established in more than a dozen fields scattered throughout Japan (Marsden and Kawai, 1965). Production in 1963 was about 1.69.times.10.sup.9 m.sup.3 (58.68.times.10.sup.6, mcf). This dry gas is in no way associated with crude oil and is an entirely separate resource. Although some production comes from depths as great as 2,000 m (6,560 ft), most production is from depths less than 1,000 m (3,280 ft). Individual wells yield up to 6,000 m.sup.3 (208,000 cf) per day, with gas-water ratios up to about 11 cf/bbl. Well diameters are generally no larger than 5 inches, and well life ranges from about 5 to 10 years, failure being attributed mainly to corrosion. All of the produced methane now goes to the chemical industry.
Commercial development of dissolved natural gas began in Italy in the eastern part of the Po delta, northeast of Ferrara, Italy, in 1939 (Reeves, 1953), from wells limited by law to depths less than 500 m (1,640 ft). This Polesine gas producing area occupies about 2,000 km.sup.2 (about 770 mi.sup.2). In 1951, 1,400 wells were producing about 753.times.10.sup.6 m.sup.3 (26.1.times.10.sup.6 mcf), about 34 percent of Italy's total annular production of natural gas. From 1943 to 1949, this area produced more natural gas than all other fields in Italy. Wells usually flow for a few years, and then must be pumped. The ultimate yield before failure is about 600,000 to 800,000 m.sup.3 (20,833 mcf to 27,700 mcf).
Gas-depleted salt water is discharged by canal or saline estuary to the sea, in both Japan and Italy. Contamination of farm lands by leaky canals has occurred in both countries, and land subsidence as a consequence of withdrawals has resulted in curtailment of production in some areas in both countries. Poor equipment, faulty technology, and haphazard operations have led to serious problems and small profit margins in both Italy and Japan.
An investigation of the natural gas content of subsurface waters was made by Humble Oil and Refining Company (now Exxon USA) during the middle 1940's (Buckely et al, 1958). Water samples were collected by specially-designed downhole tools and carefully analyzed in the laboratory. Samples were taken from some 300 wells distributed from New Mexico to Florida, but concentrated primarily in East Texas, the upper Gulf Coast of Texas, and southern Mississippi. The primary objective of this investigation was to discover whether or not dissolved gaseous hydrocarbons exist generally in subsurface waters, and, if they do, to determine the extent of their distribution and the manner in which the distribution might be affected locally by accumulations of oil or gas in the same formation, in deeper formations, or in shallower formations. Results show that at depths below a few thousands of feet, saline formaton waters contain measurable amounts of dissolved natural gas, primarily methane; that the natural gas/methane content generally increases with depth; that in formations older than the Oligocene in the areas and depth ranges studied, percent saturation in natural gas is generally only a few percent; and that "throughout the region sampled, the Frio (Oligocene) water was found to be either saturated or nearly saturated with dissolved gas in nearly every well sampled. The total quantity of gas dissolved in the water of the subsurface formations in this area probably exceeds the known proved gas reserves heretofore discovered in commercial accumulations in the area." (Pp. 881-882).
Source wells for water-flood operations in two tracts of the outer continental shelf, Gulf of Mexico, ranging in depth from about 1,400 to about 6,000 ft, yield formation water saturated in natural gas, primarily methane. Four wells, 3,200 to about 6,000 ft deep, are located in Grand Isle Bl. 16, operated by Exxon, USA; one well about 1,400 ft deep, located in Eugene Island Area Bl. 331, is operated by Shell Oil Company. Dissolved gas content is 14 to 16 cf/bbl, in water produced. Many thousands of drill-stem tests, as well as innumerable Schlumberger wire-line formation tests, confirm the presence of natural gas, primarily methane at or near saturation in saline-water aquifers throughout coastal Louisiana and Texas, onshore and offshore, below depths of a few thousands of feet.
The very great solubility of methane in water at high pressures and temperatures, as shown in FIG. 1 (Sultanov et al, 1972), and the abundant evidence for methane saturation of saline formation waters between depths of 1,400 and 20,000 ft or more, support the claim of this patent that natural gas can be produced from saline water aquifers in this depth range, in geologically young sedimentary basins in which petroleum hydrocarbons are in the process of maturation, such as the northern Gulf of Mexico basin. The dissolved methane can be produced with water discharged from wells tapping the aquifers, as in Japan and Italy, or separately from the produced water by methods described in this patent.
The amount of gas released from solution with incremental reductions of fluid pressure and/or temperature are indicated in Table 1.
TABLE 1 ______________________________________ Solubility of methane in water at selected temperatures and pressures, in standard cubic feet per barrel. (values approximate) Pressure Temperature .degree.F. psi 200 300 400 500 600 656 ______________________________________ 2,000 10 12 20 30 17 3,000 13 17 30 52 80 4,000 15 23 40 76 135 6,000 20 29 52 105 230 380 8,000 24 35 64 130 285 440 10,000 28 41 77 149 340 620 12,000 47 86 168 400 800 14,000 53 95 186 440 900 16,000 58 104 200 480 1,000 ______________________________________
These data and the curves in FIG. 1 support the observation of Perry (1972) that "the larger percentage of economical reserves (found to occur) at the higher pressure gradients reverses the previous concepts that geopressured reservoirs would contain small volumes of reserves." Unit decline of fluid pressure releases far greater amounts of gas (from water solution) at pressures between 4,000 and 12,000 psi, and at temperatures above 300.degree. F., than at lower pressures and temperatures. At 400.degree. F., volumes released by unit pressure drop are double those at 300.degree. F.; at 500.degree. F., they are quadruple; and at 600.degree. F., they are an order of magnitude greater. Such releases of dissolved methane from high-temperature, high-pressure water associated with abnormally pressured (geopressured) natural gas reservoirs in believed to explain the two distinct slopes evident in plots of shut-in bottom-hole pressures versus cumulative production (P/Z plot). Hammerlindl (1971) explains this change of slope, initially gentle and later steep, as the combined effect of changes due to gas expansion, formation compaction, crystal (rock) expansion, and water expansion. No mention is made of the effects of dissolved gas exsolution.
The origin of the nonassociated natural gas in geopressured gas reservoirs of the Gulf Basin--6,600 of which produced some 6 Tcf of natural gas in Louisiana in 1977--is discussed by Jones (1975), who attributes the gas to natural thermal cracking of all petroleum that fails to escape from the geopressure zone, supersaturating the associated formation waters. Jones has estimated (1976) that the dissolved natural gas resource of the northern Gulf of Mexico basin, in geopressured sand-bed aquifers beneath an area of some 150,000 mi.sup.2, and above a depth of 25,000 ft, is about 49,000 Tcf. Isokrari (1976), on the basis of computer studies of the production of multiphase fluids (natural gas, gas in solution, and water) concludes that water wells completed in geopressured reservoirs would be capable of delivering as much natural gas per day as many conventional gas wells. "Parametric studies of cost of producing natural gas as the value of individual reservoir parameters is varied reveal maximum sensitivity to those parameters most difficult to quantify," according to Randolph (1977), who concludes that (1) Reservoir criteria for natural gas production are much less stringent than for electricity generation from geopressured Gulf Coast aquifers, and (2) Large quantities of natural gas may be producible at a cost competitive with alternative sources from aquifers whose producing characteristics are sub-marginal for supporting investment in facilities to generate electricity from thermal and mechanical energy.
In Jones (publication 8 above), I describe the basic principles upon which the present invention is based. More particularly, I disclose that natural gas contained in the waters of geopressured and hydropressured aquifers of the northern Gulf of Mexico basin can be recovered by withdrawing water from the aquifer. Withdrawal of the water reduces the pressure within the aquifer and thus causes the natural gas originally dissolved in the water at or near saturation levels to exsolve from the water and commence free flow to form a gas cap. The gas is then capable of being withdrawn or recovered from the aquifer essentially water-free. Recovery of the exsolved gas and continued removal of the water causes further exsolution of additional quantities of dissolved gas which allows for continuous recovery of water-free gas (except for water vapor). This process continues until most of the dissolved gas has exsolved. However, no method or apparatus is disclosed for accomplising the withdrawal of the aquifer water to cause gas flow and permit simultaneous recovery of the water-free gas.
Patents considered related to this invention are as follows (in descending order of estimated relevancy).
U.S. Pat. Nos. 4,040,487 and 4,042,034 have identical specifications and drawings, and both relate to a process for producing natural gas which is unrecoverable by conventional methods. In applying the method to an appropriate geopressured reservoir, water is produced at a rate sufficient to lower the aquifer pressure and thereby release gas which will migrate and be produced. It is disclosed that it is desirable and necessary to produce water from wells at a very high production rate so as to reduce the formation pressure significantly and preferably as quickly as possible throughout as large an extent of the aquifer as possible. Due to this lowering of the aquifer pressure, gas will be released from solution with the water, will expand and join either the free gas phase dispersed in the water within the sand pores or the free gas present in a gas cap. It may even form a new gas cap if far enough removed from the well so that gravitational forces overcome differential pressure forces which normally cause the gas to flow toward the well. Because natural gas flows more easily through a porous formation than does water, gas will migrate if concentrations greater than residual gas exist. The residual gas concentration will be joined by released gas or expanded gas in the reservoir, and will come to the well bore to be produced with the water which also contains its solution gas. If the producing well is located in a formation close to a free gas phase attic, the lowering of the aquifer pressure can also cause the attic gas to expand and be produced at the well bore as the gas displaces the water and cones into the producing well. Condensate contained in the attic gas would additionally be produced along with the water and gas. A free gas cap remote from the producing well may be created or enlarged and it may be prudent to produce these areas in order to increase gas recovery from the reservoir and thereby to extract the maximum quantity of gas from it.
The reserves of gas contained in geopressured aquifers are speculative due to the scarcity of data regarding the aquifer location, size and gas concentration. It is probable that the first targets for producing gas using the method of U.S. Pat. Nos. 4,040,487 and 4,042,034 will be geopressured water drive gas reservoirs which have been produced to the maximum extent with conventional methods. The principal reason for choosing this type of reservoir is that there is a known free gas phase dispersed in the water within the rock pores and a known degree of geopressure. Additionally, the presence of existing wells which can be used for producing water or injecting it into shallower sands will enhance the economics of such a project. A second type of reservoir which is a candidate for this method is a geopressured reservoir which has indicated free gas on logs, which would not produce water-free.
An ideal candidate reservoir for gas production by this method should have:
(1) A high degree of geopressure and strong water drive.
(2) A moderate resistance to the flow of water and gas through a range of permeability from 30 to 200 millidarcies.
(3) A history of produced gas, i.e. a free gas phase dispersed in water within the pores of the rock.
(4) Existing gas wells in the formation which are still usable for either production or reinjection of water.
(5) A shallow salt water formation for disposal of produced water.
(6) Attic gas upstructure in the reservoir, remaining after cessation of production by conventional means.
(7) A high condensate to gas ratio in the attic.
U.S. Pat. Nos. 3,258,069 and 3,330,356, relate to a method and apparatus, respectively, for tapping the aqueous liquids in geopressured aquifers. There is no mention of dissolved hydrocarbons.
U.S. Pat. No. 3,330,356 is a continuation-in-part of U.S. Pat. No. 3,258,069, and further discloses the recovery of petroleum light hydrocarbons, contained in the aqueous liquids brought up to the well head.
Other patents which are known, but which appear to be less relevant than the above, include the following (in numerical order).
U.S. Pat. No. 1,272,625 relates to oil wells, which may contain gas. There is a disclosure of a coaxial inner tube, and the separation of the oil from the gas, but not in an analogous manner to the subject invention.
U.S. Pat. No. 2,077,912 relates to gas wells. There is a discussion of prior art and methods for removal of undesirable water and a disclosure of removing only the gas from a flooded well, using a coaxial tube and a removable submerged plug.
U.S. Pat. No. 2,230,001 relates to oil wells. It discloses tapping water in a separate well from a separate stratum, compressing and filtering the water above ground, and pumping it into an oil well under pressure, using a coaxial tube.
U.S. Pat. No. 2,258,615 relates to oil wells containing water. It discloses introducing crude oil into the oil well, to stratify the oil and water, using a coaxial tube.
U.S. Pat. No. 2,736,381 relates to wells containing normally liquid hydrocarbons (oil) in a gaseous phase, mixed with methane. It discloses using two wells: the first connecting a high pressure stratum with a lower pressure upper stratum, and sealed at the well head; the second tapping the lower pressure stratum, whose pressure is increased by the first well. There is no water removal.
U.S. Pat. No. 2,760,578 relates to obtaining oil and gas from different strata. It discloses a removable inner flow tube which may be raised or lowered, with oil going up the main bore and gas up the tube, or the reverse.
U.S. Pat. No. 3,123,134 relates to oil wells. It discloses a method of recovering additional oil from watered-out reservoirs by gas injection into surrounding wells.
U.S. Pat. No. 3,134,438 relates to oil wells. It discloses an inner tube inside a well, but for a different purpose and used differently from the subject invention.
U.S. Pat. No. 3,177,940 relates to a method for obtaining fresh water from brine, using a well with an inner tube.
U.S. Pat. No. 3,215,198 relates to gas wells. It discloses a method for pressure maintenance by gas injection.
U.S. Pat. No. 3,302,581 relates to gas wells. It discloses water removal by the use of a collapsible plug injected into the well, which is lifted by the gas pressure.